xxx.ts

  @Field({
    type: () => TopicEnrolmentModel,
    description: `creates a new enrolment if retries are allowed`
  })
  async reEnrol(
    @Context ctx: UserSessionModel
  ) {
    const tv = await this.$relatedQuery('currentTopicVersion')

    if (tv.allow_retry) {
      const enrolment = await this.createEnrolment(ctx.user, tv)
      return enrolment
    } else {
      return null
    }
  }

  async selfEnrol(user: UserModel) {
    const tv = await this.$relatedQuery('currentTopicVersion')
    if (!tv) {
      throw new InvalidOperation(`topic ${this.id} is not published`) // no user can self enrol to an unpublished topic
    }
    const enrolment = await this.createEnrolment(user, tv)
    return enrolment
  }

  private async createEnrolment(user: UserModel, topicVersion: TopicVersionModel) {
    const enrolment = await this.$relatedQuery<TopicEnrolmentModel>(
      'enrolments'
    ).insertAndFetch({
      topic_version_id: this.production_topic_version_id,
      user_id: user.id,
      topic_id: this.id,
      start_date: new Date(),
      shared_by: null,
      available_points: topicVersion.number_quizzes,
      pass_mark: topicVersion.pass_mark,
      is_assigned: false
    })
    return enrolment
  }

TopicModel or TopicModelAsStudent ?